The invention relates to transistor oscillators in general and to CMOS circuits in particular. It has been found that in order to operate such oscillators at high frequencies, large area transistors must be used and they tend to draw large power supply currents. When low frequency operation is desired smaller lower current transistors will suffice. The simplest oscillator comprises an inverter amplifier that has sufficient gain to overcome the losses in a tuned circuit that couples the output back to the input. In this configuration the tuned circuit provides a phase inversion so that the inverter plus tuned circuit combination is oscilatory. One of the best known combinations involves a CMOS inverter with a quartz crystal coupled from the output back to the input. When the crystal is operated in its parallel resonance mode the required signal inversion is available.
In general, the higher the frequency of operation the higher the CMOS inverter gain required. This means that the device size must be determined for the frequency of the application. It also means that when the device is operated at a lower than design frequency the large devices will draw more power supply current than needed. This can be undesirable particularly for battery operated applications.
In the prior art it has been recognized that while a CMOS oscillator can be made to run very efficiently, the use of relatively small area devices will result in a circuit that is difficult to start up upon the application of power. Such a condition is disclosed in U.S. Pat. No. 4,218,661 which issued to Yoichi Imamura on Aug. 19, 1980. A CMOS oscillator is disclosed along with means to increase the closed loop gain when it is desired to start the circuit. The teaching in this patent is incorporated herein by reference.